Consider two triads of points, a, b, c, a', b', c' in a plane ; then, using homogeneous coordinates and regarding the points as specified by the corresponding vectors, we may write

and writing b × c = α, c × a = β, a × b = γ then a × a' = m1γ − n1β, b × b' = n2α − l2γ, c × c' = l3β − m3α.